Rubber Composition and Pneumatic Tire Using Same

ABSTRACT

Provided is a rubber composition containing a diene rubber including a styrene-butadiene copolymer component including at least one type of styrene-butadiene copolymer and a reinforcing filler. The bonded styrene content is from 5 to 50 wt. %. The total amount of styrene of an ozone decomposed component S1 including one styrene-derived unit and an ozone decomposed component S1V1 including one styrene-derived unit and one 1,2-bonded butadiene-derived unit is less than 80 wt. % of the amount of bonded styrene. The total amount of styrene of the decomposed component S1V1 is not less than 10 wt. % of the amount of bonded styrene. The integrated intensity of an ozone decomposed component S1V2 including one styrene-derived unit and two 1,2-bonded butadiene-derived units is not less than 15% of the integrated intensity of all decomposed components including styrene-derived units. The vinyl content of a butadiene portion is not less than 20% and less than 50%.

TECHNICAL FIELD

The present technology relates to a rubber composition configured so asto enhance low heat build-up and tensile elongation at break to orbeyond conventional levels, and a pneumatic tire using the same.

BACKGROUND ART

In recent years, there has been a demand for high wet grip performanceand low rolling resistance in pneumatic tires. In order to satisfy thesedemands, there is a known technique of compounding a reinforcing fillersuch as a styrene-butadiene copolymer or silica with a rubbercomposition constituting a cap tread of a tire. In order to furtherenhance the wear resistance or rubber hardness of the rubbercomposition, it has been proposed to compound polybutadiene or a silicahaving high reactivity, for example, but in this case, there has been aproblem in that the rubber strength decreases or the processability isdiminished.

Japanese Unexamined Patent Application Publication No. 03-239737describes that a pneumatic tire using a rubber composition prepared bycompounding a silica and a styrene-butadiene copolymer having a specificarrangement of styrene-derived units for a tread simultaneously achieveswet skid resistance, rolling resistance, and wear resistance. However,demand for a decrease in rolling resistance is gradually increased, andthe technology described in Japanese Unexamined Patent ApplicationPublication No. 03-239737 does not always sufficiently satisfy thedemand.

Japanese Unexamined Patent Application Publication No. 57-179212describes a styrene-butadiene copolymer in which, a long-chain styreneblock content is not greater than 5 wt. %, a simple chain content havingone styrene-derived unit is not less than 50 wt. % relative to the totalstyrene content in the styrene-butadiene copolymer, and a total styrenecontent is from 10 to 30 wt. % of the styrene-butadiene copolymercontent. However, this technology is not sufficient to enhance the lowheat build-up in the rubber composition.

SUMMARY

The present technology provides a rubber composition configured so as toenhance low heat build-up and tensile elongation at break to or beyondconventional levels.

The rubber composition of the present technology which achieves theobject described above is a rubber composition including a diene rubbercontaining at least one type of styrene-butadiene copolymer and areinforcing filler, the styrene-butadiene copolymer component includingthe at least one type of styrene-butadiene copolymer having thecharacteristics of (1) to (4) below:

(1) a bonded styrene content is from 5 to 50 wt. %;

(2) when a decomposed component S1 including one styrene-derived unitand a decomposed component S1V1 including one styrene-derived unit andone 1,2-bonded butadiene-derived unit are measured by gel permeationchromatography as decomposed components obtained by ozone decomposition,a total amount of styrene of the decomposed component S1 and thedecomposed component S1V1 is less than 80 wt. % of the amount of bondedstyrene, and a total amount of styrene of the decomposed component S1V1is not less than 10 wt. % of the amount of bonded styrene;

(3) when the decomposed components obtained by ozone decomposition aremeasured by liquid chromatography-mass spectrometer, an integratedintensity of a decomposed component S1V2 including one styrene-derivedunit and two 1,2-bonded butadiene-derived units is not less than 15% ofan integrated intensity of all decomposed components includingstyrene-derived units; and

(4) a vinyl content of a butadiene portion is not less than 20% and lessthan 50%.

In accordance with the configuration described above, the rubbercomposition of the present technology contains a diene rubber includinga styrene-butadiene copolymer component and a reinforcing filler,wherein the styrene-butadiene copolymer component satisfies that (1) thebonded styrene content is from 5 to 50 wt. %; (2) the total amount ofstyrene of an ozone decomposed component S1 including onestyrene-derived unit and an ozone decomposed component S1V1 includingone styrene-derived unit and one 1,2-bonded butadiene-derived unit isless than 80 wt. % of the amount of bonded styrene, and the total amountof styrene of the decomposed component S1V1 is not less than 10 wt. % ofthe amount of bonded styrene; (3) the integrated intensity of an ozonedecomposed component S1V2 including one styrene-derived unit and two1,2-bonded butadiene-derived units is not less than 15% of theintegrated intensity of all decomposed components includingstyrene-derived units; and (4) the vinyl content of a butadiene portionis not less than 20% and less than 50%. Therefore, the low heat build-upand tensile elongation at break can be enhanced to or beyondconventional levels. Further, the wear resistance of the rubbercomposition can be improved.

The diene rubber may contain at least one type selected from naturalrubber, polyisoprene, and polybutadiene. The reinforcing filler may beat least one type selected from silica and carbon black.

The rubber composition described above is suitable for use in apneumatic tire and is particularly preferably used in a cap tread. Thispneumatic tire may have enhanced wear resistance and low rollingresistance to or beyond conventional levels.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a partial cross-sectional view in a tire meridian directionthat illustrates an example of an embodiment of a pneumatic tire inwhich a rubber composition of the present technology is used.

DETAILED DESCRIPTION

FIG. 1 is a cross-sectional view that illustrates an example of anembodiment of a pneumatic tire in which a rubber composition is used.The pneumatic tire includes a tread portion 1, a sidewall portion 2, anda bead portion 3.

In FIG. 1, two carcass layers 4, formed by arranging reinforcing cordsextending in a tire circumferential direction at a predetermined pitchand embedding these reinforcing cords in a rubber layer, are disposedextending between left and right bead parts 3. Both ends of the carcasslayers 4 are made to sandwich a bead filler 6 and are folded back arounda bead core 5 that is embedded in the bead parts 3 in a tire axialdirection from the inside to the outside. An innerliner layer 7 isdisposed inward of the carcass layers 4. Two layers of a belt layer 8,formed by arranging reinforcing cords extending inclined to the tirecircumferential direction in the tire axial direction at a predeterminedpitch and embedding these reinforcing cords in a rubber layer, isdisposed on an outer circumferential side of the carcass layers 4 of thetread portion 1. The reinforcing cords of the two layers of a belt layer8 are inclined with respect to the tire circumferential direction, andthe direction of the cords of the different layers have an oppositeorientation and cross each other. A belt cover layer 9 is disposedoutward of the belt layers 8. The tread portion 1 is formed from treadrubber layers 10 a and 10 b on the outer circumferential side of thebelt cover layer 9. The tread rubber layers 10 a and 10 b are a captread and a base tread and are preferably made of the rubber compositionof the present technology.

The rubber composition of the present technology contains a diene rubberand a reinforcing filler. The diene rubber includes at least one type ofstyrene-butadiene copolymer. In the present specification, a polymercomponent made of at least one type of styrene-butadiene copolymer maybe called a “styrene-butadiene copolymer component”. In the presenttechnology, the styrene-butadiene copolymer component satisfies all ofthe characteristics of (1) to (4) below:

(1) the bonded styrene content is from 5 to 50 wt. %;

(2) when a decomposed component S1 including one styrene-derived unitand a decomposed component S1V1 including one styrene-derived unit andone 1,2-bonded butadiene-derived unit are measured by gel permeationchromatography (GPC) as decomposed components obtained by ozonedecomposition, the total amount of styrene of the decomposed componentS1 and the decomposed component S1V1 is less than 80 wt. % of the amountof bonded styrene, and the total amount of styrene of the decomposedcomponent S1V1 is not less than 10 wt. % of the amount of bondedstyrene;

(3) when the decomposed components obtained by ozone decomposition aremeasured by liquid chromatography-mass spectrometer (LCMS), theintegrated intensity of a decomposed component S1V2 including onestyrene-derived unit and two 1,2-bonded butadiene-derived units is notless than 15% of the integrated intensity of all decomposed componentsincluding styrene-derived units; and

(4) the vinyl content of a butadiene portion is not less than 20% andless than 50%.

When the styrene-butadiene copolymer component includes a singlestyrene-butadiene copolymer, the single styrene-butadiene copolymerneeds to satisfy all of the characteristics of (1) to (4) describedabove.

In addition, when the styrene-butadiene copolymer component includes ablend of a plurality of styrene-butadiene copolymers, thestyrene-butadiene copolymer component must satisfy all of thecharacteristics of (1) to (4) described above as a whole. As long as thestyrene-butadiene copolymer component satisfies the characteristics of(1) to (4) as a whole, each styrene-butadiene copolymer constituting theblend may or may not satisfy all of the characteristics of (1) to (4)described above. Each of the styrene-butadiene copolymers constitutingthe blend preferably satisfy all of the characteristics of (1) to (4).By forming the styrene-butadiene copolymer component from two or moretypes of styrene-butadiene copolymers which satisfy all of thecharacteristics (1) to (4), the low heat build-up and tensile elongationat break of the rubber composition can be further improved. Moreover,the wear resistance of the rubber composition can be improved.

In the present technology, the styrene-butadiene copolymer satisfies(1): the bonded styrene content is from 5 to 50 wt. % and preferablyfrom 10 to 40 wt. %. When the styrene content of the styrene-butadienecopolymer component falls within such a range, the balance between thewear resistance and rubber strength of the rubber composition and thewet skid characteristics can be improved. When the styrene content ofthe styrene-butadiene copolymer component is less than 5 wt. %, the wetskid characteristics, wear resistance, and rubber strength aredeteriorated. When the styrene content of the styrene-butadienecopolymer exceeds 50 wt. %, the glass transition temperature (Tg) of thestyrene-butadiene copolymer rises, the balance of viscoelasticcharacteristics is diminished, and the effect of reducing heat build-upbecomes difficult to achieve. That is, the balance between hysteresisloss and wet skid characteristics is diminished. Note that the styrenecontent of the styrene-butadiene copolymer is measured by ¹H-NMR.

The styrene-butadiene copolymer used in the present technology satisfies(2): an ozone decomposed component S1 including one styrene-derived unitand an ozone decomposed component S1V1 including one styrene-derivedunit and one 1,2-bonded butadiene-derived unit are measured by gelpermeation chromatography (GPC) as decomposed components obtained byozone decomposition. At that time, the total amount of styrene of theozone decomposed component S1 and the ozone decomposed component S1V1 isless than 80 wt. % of the amount of bonded styrene, and the total amountof styrene of the ozone decomposed component S1V1 is not less than 10wt. % of the amount of bonded styrene.

The styrene-butadiene copolymer is a copolymer of styrene and butadieneand includes repeating units of styrene (styrene units) and repeatingunits of butadiene (butadiene units). The butadiene units include aportion in which butadiene is polymerized by 1,2-bonds (repeating unitsof ethylene having a vinyl group in a side chain) and a portion in whichbutadiene is polymerized by 1,4-bonds (repeating units of divalentgroups of 2-butylene). In addition, the portion polymerized by 1,4-bondsincludes repeating units with a trans-2-butylene structure and repeatingunits with a cis-2-butylene structure.

When the styrene-butadiene copolymer is subjected to ozonedecomposition, the portion polymerized by 1,4-bonds is cleaved. Inaddition, the vinyl group of the side chain is oxidized to form ahydroxymethyl group. As a result, the repeating units sandwiched betweentwo adjacent butadiene units polymerized by 1,4-bonds in thestyrene-butadiene copolymer are produced as ozone decomposed components.For example, when a portion in which only one styrene unit in the mainchain is sandwiched between two butadiene units polymerized by 1,4-bondsis subjected to ozone decomposition, a compound represented by thegeneral formula (I) below is produced. In the present specification, thecompound represented by the general formula (I) is called “ozonedecomposed component S1”.

In addition, when a portion in which one styrene unit in the main chainand one butadiene unit polymerized by a 1,2-bond are sandwiched byadjacent butadiene units polymerized by 1,4-bonds is subjected to ozonedecomposition, compounds represented by the general formulas (II) and(III) below are produced. In the present specification, the compoundsrepresented by the general formulas (II) and (III) are called “ozonedecomposed components S1V1”.

Further, when a portion in which one styrene unit in the main chain andtwo butadiene units polymerized by 1,2-bonds are sandwiched by adjacentbutadiene units polymerized by 1,4-bonds is subjected to ozonedecomposition, compounds represented by the general formulas (IV) to(VI) below are produced. In the present specification, the compoundsrepresented by the general formulas (IV) to (VI) are called “ozonedecomposed components S1V2”.

The portion sandwiched between two adjacent butadiene units polymerizedby 1,4-bonds as described above is produced as a decomposed component inwhich a styrene-derived unit and/or a 1,2-bonded butadiene-derived unitare sandwiched by hydroxyethyl groups at both terminals. In addition,1,4-butanediol is produced from repeating portions having two or moreconsecutive butadiene units polymerized by 1,4-bonds.

When the decomposed components obtained by ozone decomposition aremeasured by gel permeation chromatography (GPC) in the styrene-butadienecopolymer component used in the present technology, the total amount ofstyrene of the ozone decomposed component S1 and the ozone decomposedcomponent S1V1 is less than 80 wt. %, preferably from 30 to 70 wt. %,and more preferably from 50 to 70 wt. % of the amount of bonded styrene.Here, decomposed components including one styrene-derived unit refer tothe ozone decomposed component S1 including only one styrene-derivedunit and the ozone decomposed component S1V1 including onestyrene-derived unit and one 1,2-bonded butadiene-derived unit asdescribed above. The number of moles of the styrene-derived units ineach decomposed component is determined by measuring the ozonedecomposed components by gel permeation chromatography (GPC). The weightof styrene in each ozone decomposed component is calculated on the basisof this number of moles of styrene-derived units. The total amount ofstyrene of the ozone decomposed components S1 and S1V1 determined inthis manner needs to be less than 80 wt. % of the amount of bondedstyrene. This makes it possible to achieve excellent wear resistance.

In addition to the above description, when the decomposed componentsobtained by ozone decomposition are measured by gel permeationchromatography (GPC) in the styrene-butadiene copolymer component usedin the present technology, the total amount of styrene of the ozonedecomposed component S1V1 including one styrene-derived unit and one1,2-bonded butadiene-derived unit is not less than 10 wt. % andpreferably from 10 to 30 wt. % of the amount of bonded styrene. Here,the ozone decomposed component S1V1 is an ozone decomposed componentincluding one styrene-derived unit and one 1,2-bonded butadiene-derivedunit as described above, and corresponds to the decomposed componentsrepresented by the general formulas (II) and (III) above. The number ofmoles of the decomposed components represented by the general formulas(II) and (III) is determined by measuring the ozone decomposedcomponents by gel permeation chromatography (GPC), and the weight ofstyrene is calculated on the basis of this number. The amount of styrenein the ozone decomposed component including one styrene-derived unit andone 1,2-bonded butadiene-derived unit needs to be not less than 10 wt. %of the amount of bonded styrene. This makes it possible to achievesuperior low heat build-up and tensile elongation at break. Further, thewear resistance can be ensured.

In the present specification, the method for subjecting thestyrene-butadiene copolymer to ozone decomposition and the measurementof ozone-decomposed products are performed in accordance with themethods described by Tanaka, et al., Polymer, 22, 1721 (1981) andMacromolecules, 16, 1925 (1983). Note that in the analysis methoddescribed by Tanaka, et al., the total of general formulas (I), (II),and (III) described above is called a “styrene simple chain”. Incontrast, as described above, in the present technology, attention isfocused on the total amount of the ozone decomposed component S1including only one styrene-derived unit and the ozone decomposedcomponent S1V1 including one styrene-derived unit and one 1,2-bondedbutadiene-derived unit (S1+S1V1; total decomposed components representedby the general formulas (I), (II), and (III) above) and the decomposedcomponent including one styrene-derived unit and one 1,2-bondedbutadiene-derived unit (S1V1; decomposed components represented by thegeneral formulas (II) and (III) above), and the analyses thereof areperformed separately.

In the present specification, the conditions for measuring the ozonedecomposed components by gel permeation chromatography (GPC) may be asfollows.

Measurement instrument: LC-9104 (manufactured by Japan AnalyticalIndustry Co., Ltd.)

Columns: two of each of JAIGEL-1H and JAIGEL-2H (both manufactured byJapan Analytical Industry Co., Ltd.) connected in series

Detectors: UV Detector 3702 (manufactured by Japan Analytical IndustryCo., Ltd.)

Differential refractometer RI Detector RI-7 (manufactured by JapanAnalytical Industry Co., Ltd.)

Eluent: chloroform

Column temperature: room temperature

In the styrene-butadiene copolymer component used in the presenttechnology, (3) when the decomposed components obtained by ozonedecomposition are measured by liquid chromatography-mass spectrometer(LCMS), the integrated intensity of a decomposed component S1V2including one styrene-derived unit and two 1,2-bonded butadiene-derivedunits is not less than 15% and preferably from 15 to 40% of theintegrated intensity of all decomposed components includingstyrene-derived units. When the integrated intensity of the decomposedcomponent S1V2 is not less than 15%, more excellent low heat build-upcan be achieved. Here, the ozone decomposed component S1V2 including onestyrene-derived unit and two 1,2-bonded butadiene-derived units is anozone decomposed component including only one styrene-derived unit andtwo 1,2-bonded butadiene-derived units, and corresponds to thedecomposed components represented by the general formulas (IV), (V), and(VI) above. By measuring these by liquid chromatography-massspectrometer (LCMS), the area intensities of peaks unique to decomposedcomponents having the molecular weights of general formulas (IV), (V),and (VI) are determined.

The integrated intensity of each decomposed component can be determinedusing the following measurement method and analysis method. Since themolecules of each decomposed component can be detected in the state of asodium adduct ion, each mass chromatogram can be extracted on the basisof the mass spectrum thereof. In the case of the decomposed componentS1V2 including one styrene-derived unit and two 1,2-bondedbutadiene-derived units, the mass spectrum of a sodium adduct ion is333.21. In the mass chromatogram of 333.21, the peak of the decomposedcomponent S1V2 is confirmed, and the integrated intensity A[S1V2] isdetermined. Similarly, the area intensities of all other decomposedcomponents including styrene-derived units are determined, and the sumA[total] is determined. The ratio of the integrated intensity A[S1V2] ofthe ozone decomposed component S1V2 including one styrene-derived unitand two 1,2-bonded butadiene-derived units to the sum A[total] of theintegrated intensities of all the decomposed components includingstyrene-derived units is calculated from the equationA[S1V2]/A[total]×100.

In the present specification, the conditions for measuring the ozonedecomposed components by liquid chromatography-mass spectrometer (LCMS)may be as follows.

Liquid chromatograph: Alliance 2695 (manufactured by Waters)

Mass spectrometer: ZQ2000 (manufactured by Waters)

Column: Hydrosphere C18 (manufactured by YMC, inner diameter: 2.0 mm,length: 150 mm, particle size: 3 μm)

Injection rate: 5 μL (approximately 10 mg/mL)

Mobile phase A: water

Mobile phase B: methanol

Flow rate: 0.2 mL/min

Time program: B conc. 20% (0 min)→100% (35 min)→100% (50 min)

Ion source temperature: 120° C.

Desolvent temperature: 350° C.

Cone voltage: 40 V

Ionization method: (ESI positive mode)

Mass spectrometry conditions: Scan measurement, mass range: m/z 50-2000

In the styrene-butadiene copolymer component used in the presenttechnology, (4) the vinyl content of the butadiene portion is not lessthan 20% and less than 50%. When the vinyl content of the butadieneportion in the styrene-butadiene copolymer component is not less than20%, the rubber strength can be maintained and the balance between thewet skid characteristics and the rolling resistance can be madefavorable. When the vinyl content of the butadiene portion in thestyrene-butadiene copolymer component is less than 50%, the tensileelongation at break can be maintained or improved. Note that the vinylcontent of the butadiene portion is measured by ¹H-NMR.

The content of the styrene butadiene copolymer component having thecharacteristics of (1) to (4) is preferably not less than 40 wt. %, morepreferably from 60 to 100 wt. %, and even more preferably from 80 to 100wt. % out of 100 wt. % of the diene rubber. When the content of thestyrene-butadiene copolymer component specified by characteristics (1)to (4) is not less than 40 wt. %, the low heat build-up and tensileelongation at break of the rubber composition can be made moreexcellent.

The styrene-butadiene copolymer component specified by characteristics(1) to (4) can be prepared by using a single styrene-butadiene copolymeror by combining a plurality of styrene-butadiene copolymers. In general,the chain structure of a styrene-butadiene copolymer synthesized by asolution polymerization method can be controlled, and thecharacteristics (1) to (4) can be easily adjusted by the polymerizationtemperature at the time of synthesis, the timing of introducingmonomers, the types and amounts of randomizers, and the like. Inaddition, when an existing styrene-butadiene copolymer is blended, thestyrene-butadiene copolymer component having characteristics (1) to (4)can be prepared by combining a plurality of solution-polymerizedstyrene-butadiene copolymers having controllable chain structures andcombining an emulsion-polymerized styrene-butadiene copolymer primarilyhaving a random structure and one or more solution-polymerizedstyrene-butadiene copolymers.

The rubber composition of the present technology may include other dienerubbers in addition to a styrene-butadiene copolymer componentsatisfying all of the characteristics (1) to (4). Examples of otherdiene rubbers include natural rubber (NR), polyisoprene rubber (IR),polybutadiene rubber (low-cis BR), high-cis BR, high-trans BR(trans-bond content of the butadiene portion: 70 to 95%),styrene-isoprene copolymer rubber, butadiene-isoprene copolymer rubber,solution-polymerized random styrene-butadiene-isoprene copolymer rubber,emulsion-polymerized random styrene-butadiene-isoprene copolymer rubber,emulsion-polymerized styrene-acrylonitrile-butadiene copolymer rubber,acrylonitrile-butadiene copolymer rubber, high-vinyl SBR/low-vinyl SBRblock copolymer rubber, polyisoprene-SBR block copolymer rubber, andpolystyrene-polybutadiene-polystyrene block copolymers.

The content of the other diene rubber is preferably not greater than 60wt. %, more preferably from 0 to 40 wt. %, and even more preferably from0 to 20 wt. % out of 100 wt. % of the diene rubber. When another dienerubber is contained, various physical properties such as wearresistance, low heat build-up, and tensile elongation at break can beimproved.

The rubber composition of the present technology contains a diene rubberand a reinforcing filler. Examples of reinforcing fillers includeinorganic fillers such as carbon black, silica, clay, aluminumhydroxide, calcium carbonate, mica, talc, aluminum hydroxide, aluminumoxide, titanium oxide, and barium sulfate and organic fillers such ascellulose, lecithin, lignin, and dendrimer. Of these, it is preferableto compound at least one type selected from carbon black and silica.

When carbon black is compounded with the rubber composition, the wearresistance and rubber strength of the rubber composition can be madeexcellent. The compounded amount of the carbon black is not particularlylimited but is preferably from 10 to 100 parts by weight and morepreferably from 25 to 80 parts by weight per 100 parts by weight of thediene rubber.

Carbon black such as furnace black, acetylene black, thermal black,channel black, and graphite may be compounded. Of these, furnace blackis preferable, and specific examples thereof include SAF, ISAF, ISAF-HS,ISAF-LS, IISAF-HS, HAF, HAF-HS, HAF-LS, and FEF. These carbon blacks mayeach be used alone, or two or more types may be used in combination. Inaddition, a surface-treated carbon black obtained by chemicallymodifying these carbon blacks with various acid compounds or the likemay also be used.

In addition, compounding silica with the rubber composition makes itpossible to obtain a rubber composition exhibiting excellent low heatbuild-up and wet grip performance. The compounded amount of the silicais not particularly limited but is preferably from 10 to 150 parts byweight and more preferably from 40 to 100 parts by weight per 100 partsby weight of the diene rubber.

Any silica that is ordinarily used in rubber compositions for a tiretread such as wet silica, dry silica, carbon-silica in which silica issupported on a carbon black surface (dual-phase filler), or silicasurface-treated with a compound which is reactive or compatible withboth silica and rubber such as a silane coupling agent or polysiloxane.Of these, a wet silica containing hydrous silicic acid as a maincomponent is preferable.

In the present technology, the compounded amount of the reinforcingfiller including silica and/or carbon black is preferably from 10 to 150parts by weight and more preferably from 40 to 100 parts by weight per100 parts by weight of the diene rubber. When the compounded amount ofthe reinforcing filler is less than 10 parts by weight, the reinforcingperformance cannot be sufficiently obtained, and the rubber hardness andtensile strength at break become insufficient. When the compoundedamount of the reinforcing filler exceeds 150 parts by weight, the heatbuild-up of the rubber composition increases while the tensileelongation at break decreases. Moreover, the wear resistance andprocessability are also diminished.

A silane coupling agent is preferably compounded with the rubbercomposition of the present technology together with silica in that thelow heat build-up and wear resistance are further enhanced. Bycompounding a silane coupling agent together with silica, thedispersibility of the silica is enhanced, and the reinforcing actionwith the diene rubber is further increased. The compounded amount of thesilane coupling agent is preferably from 2 to 20 wt. % and morepreferably from 5 to 15 wt. % of the compounded amount of the silica.When the compounded amount of the silane coupling agent is less than 2wt. % of the weight of the silica, the effect of improving thedispersibility of the silica cannot be sufficiently obtained.Additionally, when the compounded amount of the silane coupling agentexceeds 20 wt. %, the diene rubber component tends to be easilygelified, so the desired effects cannot be achieved.

The silane coupling agent is not particularly limited, but asulfur-containing silane coupling agent is preferable, and examplesthereof include bis(3-triethoxysilylpropyl)tetrasulfide,bis(3-triethoxysilylpropyl)trisulfide,bis(3-triethoxysilylpropyl)disulfide,bis(2-triethoxysilylethyl)tetrasulfide,bis(3-trimethoxysilylpropyl)tetrasulfide,bis(2-trimethoxysilylethyl)tetrasulfide, mercaptosilane compounds andthe like described in JP-2006-249069 A such as3-mercaptopropyltrimethoxysilane, 3-mercaptopropyldimethoxymethylsilane,3-mercaptopropyldimethylmethoxysilane, 2-mercaptoethyltriethoxysilane,3-mercaptopropyltriethoxysilane, and VP Si363 manufactured by Evonik,3-trimethoxysilylpropylbenzothiazole tetrasulfide,3-triethoxysilylpropylbenzothiazolyl tetrasulfide,3-triethoxysilylpropylmethacrylate monosulfide,3-trimethoxysilylpropylmethacrylate monosulfide,3-trimethoxysilylpropyl-N,N-dimethylthiocarbamoyl tetrasulfide,3-triethoxysilylpropyl-N,N-dimethylthiocarbamoyl tetrasulfide,2-triethoxysilylethyl-N,N-dimethylthiocarbamoyl tetrasulfide,bis(3-diethoxymethylsilylpropyl)tetrasulfide,dimethoxymethylsilylpropyl-N,N-dimethylthiocarbamoyl tetrasulfide,dimethoxymethylsilylpropylbenzothiazolyl tetrasulfide,3-octanoylthiopropyltriethoxysilane,3-propionylthiopropyltrimethoxysilane, vinyl trimethoxysilane, vinyltriethoxysilane, vinyl tris(2-methoxyethoxy)silane,3-glycidoxypropyltrimethoxysilane,3-glycidoxypropylmethyldimethoxysilane,β-(3,4-epoxycyclohexyl)ethoxytrimethoxysilane,3-aminopropyltrimethoxysilane,N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane, andN-(β-aminoethyl)-γ-aminopropylmethyldimethoxysilane. In addition, thesilane coupling agent may be an organosilicon compound, and examples oforganosilicon compounds include polysiloxanes, silicon oils into whichone or more organic groups such as an amino group, an epoxy group, acarbinol group, a mercapto group, a carboxyl group, a hydrogen group, apolyether group, a phenol group, a silanol group, an acrylic group, amethacrylic group, or a long-chain alkyl group are introduced into aside chain, both terminals, one terminal, or a side chain and bothterminals of a polysiloxane, and silicon oligomers obtained byperforming a condensation reaction on one or more type of organicsilanes. Of these, bis-(3-triethoxysilylpropyl)tetrasulfide andbis(3-(triethoxysilyl)propyl) disulfide are preferable.

In addition to the components described above, the rubber composition ofthe present technology may also include various compounding agents thatare commonly used in rubber compositions for a tire tread. Examplesthereof include vulcanizing or cross-linking agents, vulcanizationaccelerators, anti-aging agents, processing aids, plasticizers, liquidpolymers, thermosetting resins, thermoplastic resins, and the like.These compounding agents are kneaded by a common method to obtain arubber composition that can be used for vulcanization or cross-linking.These compounding agents can be compounded in conventional generalamounts so long as the object of the present technology is not hindered.The rubber composition for a tire tread can be prepared by mixing theabove-mentioned components using a well-known rubber kneading machinesuch as a Banbury mixer, a kneader, or a roller.

The vulcanizing or cross-linking agent is not particularly limited, butexamples thereof include sulfur such as powdered sulfur, precipitatedsulfur, colloidal sulfur, insoluble sulfur, and highly dispersiblesulfur; halogenated sulfur such as sulfur monochloride and sulfurdichloride; and organic peroxides such as dicumyl peroxide anddi-tert-butyl peroxide. Of these, sulfur is preferable, and powderedsulfur is particularly preferable. These vulcanizing or cross-linkingagents may each be used alone, or two or more types may be used incombination. The compounding ratio of the vulcanizing agent isordinarily from 0.1 to 15 parts by weight, preferably from 0.3 to 10parts by weight, and even more preferably from 0.5 to 5 parts by weightper 100 parts by weight of the diene rubber.

The vulcanization accelerator is not particularly limited, but examplesthereof include sulfenamide-based vulcanization accelerators such asN-cyclohexyl-2-benzothiazylsulfenamide,Nt-butyl-2-benzothiazolsulfenamide,N-oxyethylene-2-benzothiazolsulfenamide, andN,N′-diisopropyl-2-benzothiazolsulfenamide; guanidine-basedvulcanization accelerators such as diphenylguanidine,di-o-tolylguanidine, and o-tolylbiguanidine; thiourea-basedvulcanization accelerators such as diethylthiourea; thiazole-basedvulcanization accelerators such as 2-mercaptobenzothiazole,dibenzothiazyldisulfide, and 2-mercaptobenzothiazole zinc salt;thiuram-based vulcanization accelerators such as tetramethylthiurammonosulfide and tetramethylthiuram disulfide; dithiocarbamic acid-basedvulcanization accelerators such as sodium dimethyldithiocarbamate andzinc diethyldithiocarbamate; and xanthogenic acid-based vulcanizationaccelerators such as sodium isopropylxanthate, zinc isopropylxanthate,and zinc butylxanthate. Of these, it is particularly preferable tocontain a sulfenamide-based vulcanization accelerator. Thesevulcanization accelerators may each be used alone, or two or more typesmay be used in combination. The compounded amount of the vulcanizationaccelerator is preferably from 0.1 to 15 parts by weight and morepreferably from 0.5 to 5 parts by weight per 100 parts by weight of thediene rubber.

The anti-aging agent is not particularly limited, but examples thereofinclude amine-based anti-aging agents such as2,2,4-trimethyl-1,2-dihydroquinoline polymers,p,p′-dioctyldiphenylamine, N,N′-diphenyl-p-phenylenediamine, andN-phenyl-N′-1,3-dimethylbutyl-p-phenylenediamine, and phenol-basedanti-aging agents such as 2,6-di-t-butyl-4-methylphenol and2,2′-methylenebis(4-methyl-6-t-butylphenol). These anti-aging agents mayeach be used alone, or two or more types may be used in combination. Thecompounded amount of the anti-aging agent is preferably from 0.1 to 15parts by weight and more preferably from 0.5 to 5 parts by weight per100 parts by weight of the diene rubber.

The processing aid is not particularly limited, but examples thereofinclude higher fatty acids such as stearic acid, higher fatty acidamides such as stearic acid amide, aliphatic higher amines such asstearyl amine, aliphatic higher alcohols such as stearyl alcohol,partial esters of fatty acids and polyhydric alcohols such as glycerinfatty acid esters, fatty acid metal salts such as zinc stearate, andzinc oxide. The compounded amount is selected appropriately, but thecompounded amounts of higher fatty acids, aliphatic higher amides,higher alcohols, and fatty acid metal salts are preferably from 0.05 to15 parts by weight and more preferably from 0.5 to 5 parts by weight per100 parts by weight of the diene rubber. The compounded amount of zincoxide is preferably from 0.05 to 10 parts by weight and more preferablyfrom 0.5 to 3 parts by weight per 100 parts by weight of the dienerubber.

The plasticizer used as a compounding agent is not particularly limited,but an aroma-based, naphthene-based, paraffin-based, or silicone-basedextender oil is selected in accordance with the application. The amountof the plasticizer used is ordinarily from 1 to 150 parts by weight,preferably from 2 to 100 parts by weight, and even more preferably from3 to 60 parts by weight per 100 parts by weight of the diene rubber.When the amount of the plasticizer used is within this range, thereinforcing agent dispersing effect, tensile strength, wear resistance,heat resistance, and the like are balanced to a high degree. Otherexamples of plasticizers include diethylene glycol, polyethylene glycol,and silicone oils.

The thermosetting resin is not particularly limited, but examplesthereof include resorcin-formaldehyde resins, phenol-formaldehyderesins, urea-formaldehyde resins, melamine-formaldehyde resins, andphenol derivative-formaldehyde resins, and more specificallythermosetting resins which are cured or polymerized by heating orapplying heat and a methylene donor such as m-3,5-xylenol-formaldehyderesins and 5-methylresorcin-formaldehyde resins, as well as other resinssuch as guanamine resins, diallylphthalate resins, vinyl ester resins,phenol resins, unsaturated polyester resins, furan resins, polyimideresins, polyurethane resins, melamine resins, urea resins, and epoxyresins.

The thermoplastic resin is not particularly limited, but examplesthereof include general-purpose resins such as polystyrene resins,polyethylene resins, polypropylene resins, polyester resins, polyamideresins, polycarbonate resins, polyurethane resins, polysulfone resins,polyphenylene ether resins, and polyphenylene sulfide resins. Otherexamples include aromatic hydrocarbon resins such asstyrene-α-methylstyrene resins, indene-isopropenyltoluene resins, andcoumarone-indene resins, dicyclopentadiene resins, hydrocarbon resinssuch as petroleum resins containing 1,3-pentadiene, pentene,methylbutene, or the like as a main raw material, alkylphenol resins,modified phenol resins, terpenephenol resins, terpene resins, andaromatic modified terpene resins.

In the rubber composition of the present technology, the low heatbuild-up, tensile elongation at break, and wear resistance are enhancedto or beyond conventional levels. Therefore, the wear resistance and lowrolling resistance (fuel economy performance) of a pneumatic tire can beenhanced to or beyond conventional levels.

The rubber composition of the present technology can be suitably usedfor a cap tread portion, an undertread portion, a sidewall portion, anda bead filler portion of a pneumatic tire, a coating rubber for a cordsuch as a carcass layer, a belt layer, or a belt cover layer, a sidereinforcing rubber layer with a crescent-shaped cross section in arun-flat tire, a rim cushion portion, or the like. In a pneumatic tireusing the rubber composition of the present technology for thesemembers, the low rolling resistance (fuel economy performance) and wearresistance can be maintained or enhanced to or beyond conventionallevels due to the enhancement of low heat build-up, tensile elongationat break, and wear resistance.

The present technology is further explained below by examples. However,the scope of the present technology is not limited to these examples.

Examples

Styrene-butadiene copolymers were used alone or mixed at eachcompounding ratio shown in Tables 1 and 2 to prepare 13 types ofstyrene-butadiene copolymer components. (1) The content of bondedstyrene, (2) the ratio of the total amount of styrene of an ozonedecomposed component S1 including one styrene-derived unit and an ozonedecomposed component S1V1 including one styrene-derived unit and one1,2-bonded butadiene-derived unit to the amount of bonded styrene(S1+S1V1; wt. %) and the ratio of the total amount of styrene of theozone decomposed component S1V1 including one styrene-derived unit andone 1,2-bonded butadiene-derived unit to the amount of bonded styrene(S1V1; wt. %); (3) the ratio of the integrated intensity of a decomposedcomponent S1V2 including one styrene-derived unit and two 1,2-bondedbutadiene-derived units to the integrated intensity of all decomposedcomponents including styrene-derived units (S1V2; %); and (4) the vinylcontent of a butadiene portion were measured. In addition, since NS460,NS522, NS570, E581, Tufdene 2330, HP755B, and Nipol 1739 are oilextended products, the net compounded amounts of the rubber componentsare shown in parentheses together with the actual compounded amounts.

The (1) content of bonded styrene and (4) vinyl content of the butadieneportion of the styrene-butadiene copolymer components were measured by¹H-NMR.

The conditions for the ozone decomposition of the styrene-butadienecopolymer components were as described above. In addition, (2) the ratioof the total amount of styrene of the ozone decomposed component S1including one styrene-derived unit and the ozone decomposed componentS1V1 including one styrene-derived unit and one 1,2-bondedbutadiene-derived unit to the amount of bonded styrene (S1+S1V1; wt. %)and the ratio of the total amount of styrene of the ozone decomposedcomponent S1V1 including one styrene-derived unit and one 1,2-bondedbutadiene-derived unit to the amount of bonded styrene (S1V1; wt. %)were measured by gel permeation chromatography (GPC). The measurementconditions for gel permeation chromatography (GPC) were as describedabove. Further, (3) the ratio of the integrated intensity of the ozonedecomposed component S1V2 including one styrene-derived unit and two1,2-bonded butadiene-derived units to the integrated intensity of alldecomposed components including styrene-derived units (S1V2; %) wasmeasured by liquid chromatography-mass spectrometer (LCMS). Themeasurement conditions for liquid chromatography-mass spectrometer(LCMS) were as described above.

For each of 13 types of rubber compositions (Examples 1 to 10 andComparative Examples 1 to 3) which commonly contained compounding agentsand contained each combination of the styrene-butadiene copolymercomponents shown in Tables 1 and 2 (blend of a plurality ofstyrene-butadiene copolymers) and another diene rubber, the componentsexcept sulfur and vulcanization accelerators were mixed for 6 minutesusing a 1.7-L sealed Banbury mixer. The mixtures were discharged fromthe mixer at 150° C., and then cooled to room temperature. Next, themixture was mixed again for 3 minutes using the 1.7-L sealed Banburymixer and discharged. The sulfur and the vulcanization accelerators werethen mixed in using an open roll to obtain a rubber composition. Theobtained rubber composition was vulcanized for 30 minutes at 160° C. ina predetermined mold to form a vulcanized rubber test piece. Using theobtained vulcanized rubber test piece, the tensile elongation at break,tan δ at 60° C., and wear resistance were evaluated by the followingmeasurement methods.

Tan δ at 60° C.

The dynamic viscoelasticity of the obtained vulcanized rubber test piecewas measured using a viscoelasticity spectrometer manufactured byIwamoto Seisakusho Co., Ltd., under conditions of elongation deformationstrain of 10±2%, a vibration frequency of 20 Hz, and a temperature of60° C., and the tan δ (60° C.) was determined. The obtained results areshown in the “tan δ (60° C.)” rows of Tables 1 and 2 as index valueswith the value of Comparative Example 1 expressed as an index of 100.Smaller index values of tan δ (60° C.) mean lower heat build-up andlower rolling resistance when a tire is produced.

Tensile Elongation at Break

Using the obtained vulcanized rubber test piece, a dumbbell JIS(Japanese Industrial Standard) No. 3 shaped test piece was produced inaccordance with JIS K6251. A tensile test was performed at a tensiletest speed of 500 mm/min at room temperature (20° C.), and the tensileelongation at break at the time of break was measured. The obtainedresults are shown in the “Tensile elongation at break” rows of Tables 1and 2 with the value of Comparative Example 1 expressed as an index of100. Greater index values of the tensile elongation at break meangreater tensile elongation at break and superior durability when a tireis produced.

Wear Resistance

Using the obtained vulcanized rubber test piece, the amount of wear wasmeasured in accordance with JIS K6264 using a Lambourn abrasion testmachine (manufactured by Iwamoto Seisakusho Co. Ltd.) under thefollowing conditions: load=15.0 kg (147.1 N), slip rate=25%. Each of thereciprocal values of the obtained results were calculated and are shownin the rows of “Wear resistance” in Tables 1 and 2 as index values usingthe reciprocal of the amount of wear of Comparative Example 1 as anindex value of 100. Greater index values of wear resistance meansuperior wear resistance.

TABLE 1 Comparative Comparative Comparative Example 1 Example 2 Example3 Tufdene 2000R part by weight 70   NS116 part by weight 30   NS522 partby weight 137.5  (100)   NS570 part by weight  81.25 (65)   NS612 partby weight 35   NR part by weight Oil part by weight 0  37.5  21.25Bonded styrene content wt. % 39.2 22.8 31.7 Vinyl content % 42.2 26.031.7 S1 + S1V1 wt. % 59.2 69.2 74.7 S1V1 wt. %  9.0  7.4  9.4 S1V2(integrated % 16.0  8.3 22.3 intensity ratio) Wear resistance Indexvalue 100   102   101   Tensile elongation at Index value 100   102  96   break tan δ (60° C.) Index value 100   131   98  

TABLE 2 Example 1 Example 2 Example 3 Example 4 Example 5 E581 part byweight  41.25 55   (30)   (40)   Tufdene 2000R part by weight 42.5Tufdene 2330 part by weight 99    96.25 (72)   (70)   YO31 part byweight 18   HPR850 part by weight 60   HP755B part by weight NS116 partby weight 42.5 NS460 part by weight NS522 part by weight NS570 part byweight 5220M part by weight 80   Nipol 1739 part by weight NR part byweight 15   BR part by weight 20   10   Oil part by weight 37.5 10.5 0 22.5 37.5 Bonded styrene wt. % 26.3 26.0 28.9 32.1 30.8 content Vinylcontent % 26.5 34.3 32.6 42.6 47.9 S1 + S1V1 wt. % 56.0 61.8 54.5 56.466.6 S1V1 wt. % 15.4 11.3 11.6 12.9 16.5 S1V2 (integrated % 36.2 20.122.1 19.1 28.2 intensity ratio) Wear resistance Index value 108   106  105   103   102   Tensile Index value 101   102   102   100   103  elongation at break tan δ (60° C.) Index value 71   95   97   91   100  Example Example 6 Example 7 Example 8 Example 9 10 E581 part by weightTufdene 2000R part by weight Tufdene 2330 part by weight YO31 part byweight HPR850 part by weight 70   HP755B part by weight  48.13 (35)  NS116 part by weight NS460 part by weight  41.25 (30)   NS522 part byweight  48.13  96.25 82.5 (35)   (70)   (60)   NS570 part by weight 87.5(70)   5220M part by weight 40   30   Nipol 1739 part by weight  41.25(30)   NR part by weight BR part by weight 30   Oil part by weight 11.24 0   26.25 15   20   Bonded styrene wt. % 39.4 35.0 30.8 34.0 36.3content Vinyl content % 40.8 48.4 46.7 35.9 30.3 S1 + S1V1 wt. % 65.755.6 58.3 57.9 64.4 S1V1 wt. % 10.4 10.1 14.8 11.6 10.4 S1V2 (integrated% 18.5 16.5 20.2 24.1 30.9 intensity ratio) Wear resistance Index value105   103   105   104   110   Tensile Index value 106   102   101  102   108   elongation at break tan δ (60° C.) Index value 95   98  88   89   88  

The types of used raw materials in Tables 1 and 2 are described below.

-   -   Tufdene 2000R: Tufdene 2000R manufactured by Asahi Kasei        Chemicals Corporation, bonded styrene content: 23.6 wt. %, vinyl        content: 9.8%, non-oil extended product    -   NS116: NS116 manufactured by Zeon Corporation, bonded styrene        content: 20.9 wt. %, vinyl content: 63.8%, non-oil extended        product    -   NS460: NS460 manufactured by Zeon Corporation, bonded styrene        content: 25.1 wt. %, vinyl content: 62.8%, oil extended product        prepared by adding 37.5 parts by weight of an oil component to        100 parts by weight of SBR    -   NS522: NS522 manufactured by Zeon Corporation, bonded styrene        content: 39.2 wt. %, vinyl content: 42.2%, oil extended product        prepared by adding 37.5 parts by weight of an oil component to        100 parts by weight of SBR    -   NS570: NS570 manufactured by Zeon Corporation, bonded styrene        content: 40.6 wt. %, vinyl content: 19.0%, oil extended product        prepared by adding 25 parts by weight of an oil component to 100        parts by weight of SBR    -   NS612: NS612 manufactured by Zeon Corporation, bonded styrene        content: 15.1 wt. %, vinyl content: 31.2%, non-oil extended        product    -   E581: E581 manufactured by Asahi Kasei Chemicals Corporation,        bonded styrene content: 35.6 wt. %, vinyl content: 41.3%, oil        extended product prepared by adding 37.5 parts by weight of an        oil component to 100 parts by weight of SBR    -   Tufdene 2330: Tufdene 2330 manufactured by Asahi Kasei Chemicals        Corporation, bonded styrene content: 25.8 wt. %, vinyl content:        28.5%, oil extended product prepared by adding 37.5 parts by        weight of an oil component to 100 parts by weight of SBR    -   YO31: YO31 manufactured by Asahi Kasei Chemicals Corporation,        Bonded styrene content: 27.1 wt. %, vinyl content: 57.5%,        non-oil extended product    -   HPR850: HPR850 manufactured by JSR Corporation, bonded styrene        content: 27.0 wt. %, vinyl content: 58.8%, non-oil extended        product    -   HP755B: HP755B manufactured by JSR corporation, bonded styrene        content: 39.6 wt. %, vinyl content: 39.4%, oil extended product        prepared by adding 37.5 parts by weight of an oil component to        100 parts by weight of SBR    -   5220M: 5220M manufactured by Korea Kumho Petrochemical Co.,        Ltd., bonded styrene content: 26.3 wt. %, vinyl content: 26.5%,        non-oil extended product    -   Nipol 1739: manufactured by Zeon Corporation, bonded styrene        content: 39.8 wt. %, vinyl content: 18.4%, oil extended product        prepared by adding 37.5 parts by weight of an oil component to        100 parts by weight of SBR    -   NR: Natural rubber, TSR20    -   BR: Polybutadiene rubber; Nipol BR1220, manufactured by Zeon        Corporation    -   Oil: Extract No. 4S, manufactured by Showa Shell Sekiyu K.K.

TABLE 3 Common formulation of rubber composition Silica 70.0 part byweight  Silane coupling agent 5.6 part by weight Carbon black 5.0 partby weight Zinc oxide 3.0 part by weight Stearic acid 2.0 part by weightAnti-aging agent 1.5 part by weight Wax 1.0 part by weight Sulfur 1.5part by weight Vulcanization accelerator 1 1.7 part by weightVulcanization accelerator 2 2.0 part by weight

The types of raw materials used as per Table 3 are described below.

-   -   Silica: Nipsil AQ manufactured by Nippon Silica Co., Ltd.    -   Silane coupling agent: sulfide-based silane coupling agent,        Si69VP manufactured by Degussa    -   Carbon black: Shoblack N339M manufactured by Showa Cabot K.K.    -   Zinc oxide: Zinc Oxide #3 manufactured by Seido Chemical        Industry Co., Ltd.    -   Stearic acid: stearic acid manufactured by NOF Corporation    -   Anti-aging agent: Santoflex 6PPD manufactured by Solutia Europe    -   Wax: paraffin wax manufactured by Ouchi Shinko Chemical        Industrial Co., Ltd.    -   Sulfur: oil-treated sulfur manufactured by Karuizawa Refinery        Ltd.    -   Vulcanization accelerator 1: Sanceller CM-PO(CZ) manufactured by        Sanshin Chemical Industry Co., Ltd.    -   Vulcanization accelerator 2: Sanceller D-G (DPG) manufactured by        Sanshin Chemical Industry Co., Ltd.

As is clear from Tables 1 and 2, it was confirmed that the low heatbuild-up of the rubber compositions of Examples 1 to 10 was enhanced.Further, it was confirmed that the wear resistance and tensileelongation at break were maintained or enhanced.

In the styrene-butadiene copolymer component of the rubber compositionof Comparative Example 2, the ratio of the total amount of styrene ofthe ozone-decomposed product including one styrene-derived unit and one1,2-bonded butadiene-derived unit to the amount of bonded styrene (S1V1)is less than 10 wt. %, and the ratio of the integrated intensity of theozone-decomposed product including one styrene-derived unit and two1,2-bonded butadiene-derived units (S1V2) is less than 15%. Therefore,the heat build-up (tan δ at 60° C.) is high.

In the styrene-butadiene copolymer component of the rubber compositionof Comparative Example 3, the ratio of the total amount of styrene ofthe ozone-decomposed product including one styrene-derived unit and one1,2-bonded butadiene-derived unit to the amount of bonded styrene (S1V1)is less than 10 wt. %. Therefore, the tensile strength at break andtensile elongation at break are inferior.

1. A rubber composition comprising a diene rubber containing at leastone type of styrene-butadiene copolymer and a reinforcing filler, the atleast one type of styrene-butadiene copolymer forming astyrene-butadiene copolymer component having characteristics (1) to (4):(1) a bonded styrene content is from 5 to 50 wt. %; (2) when adecomposed component S1 including one styrene-derived unit and adecomposed component S1V1 including one styrene-derived unit and one1,2-bonded butadiene-derived unit are measured by gel permeationchromatography as decomposed components obtained by ozone decomposition,a total amount of styrene of the decomposed component S1 and thedecomposed component S1V1 is less than 80 wt. % of the amount of bondedstyrene, and a total amount of styrene of the decomposed component S1V1is not less than 10 wt. % of the amount of bonded styrene; (3) when thedecomposed components obtained by ozone decomposition are measured byliquid chromatography-mass spectrometer, an integrated intensity of adecomposed component S1V2 including one styrene-derived unit and two1,2-bonded butadiene-derived units is not less than 15% of an integratedintensity of all decomposed components including styrene-derived units;and (4) a vinyl content of a butadiene portion is not less than 20% andless than 50%.
 2. The rubber composition according to claim 1, whereinthe diene rubber further contains at least one type selected fromnatural rubber, polyisoprene, and polybutadiene.
 3. The rubbercomposition according to claim 1, wherein the reinforcing filler is atleast one type selected from silica and carbon black.
 4. A pneumatictire using the rubber composition according to claim
 1. 5. The pneumatictire according to claim 4, wherein the rubber composition is used in acap tread.
 6. The rubber composition according to claim 2, wherein thereinforcing filler is at least one type selected from silica and carbonblack.
 7. A pneumatic tire using the rubber composition according toclaim
 6. 8. The pneumatic tire according to claim 7, wherein the rubbercomposition is used in a cap tread.